I can’t imagine these would be good eating laying on the creek bank in this heat. But freshwater mussels of the inland river and creek variety are pretty tasty if you’re lucky enough to find some in a good season. They certainly aren’t as prolific as in previous years. Changes to waterways, chemical run-off and increasing hot, dry weather – mussels like cool, fresh water, have all lead to a decrease in numbers. Where once we could scratch them up out of the sand on a river or creek bed they’re now pretty difficult to come across unless you head to Far North Australia 
If you do go mussel foraging bear in mind that its a bit like picking wild mushrooms. Do your research before you set forth! Stagnant, warm water can lead to a build up of toxins which no amount of purging can clean. And don’t choose the biggest ones because these babies are chewy at the best of times and if you don’t like the taste of clams, then they probably won’t appeal. 
Mussels formed a big part of early indigenous Australian’s diet in some areas. The shell middens they left behind proof of the importance of this food to their hunter gatherer lifestyle.  While researching in the Northern Territory for Stone Country I read quite a few accounts of first Australians often re-burying the mussels they collected in the cool, moist sand of waterways. That way the mussels would continue growing, couldn’t be foraged by animal predators and there was a store of food waiting when they next traveled through a particular area.
Cooking freshwater mussels is relatively easy. Drop them in some cold water to purge them. A minimum of thirty minutes helps the mussel get rid of grit, sand and bacteria. You can then steam the shells in water so that they open up. (Remember that if the shells don’t pop open, throw them out). Crack off the lid at the hinge and put the mussels sitting in their shells on a grill. Add some butter and garlic and serve with some chili sauce and crusty bread. Enjoy. And there’s always the supermarket or a restaurant if you’re more urban bound.